Photo-detectors are essential for many optoelectronic applications, for example, optical communication, sensing, monitoring and/or control. Photo-detectors convert optical signals into electrical signals. The electrical signals are typically in the form of electrical current which is amplified before processing for information extraction. A typical photo-detector comprises a photo-sensitive area on which incoming optical signals impinge and where conversion of optical energy into electrical energy takes place. The photo-sensitive area is normally larger than the spatial spread of an incoming light beam in order to achieve maximum responsivity. In many applications, a plurality of photo-detectors or arrays of photo-detectors are required. For example, arrays of photo-detectors are required in spectrometers.
Metal-semiconductor-metal (MSM) photo-detectors have been widely used for light detection in fibre-optic systems for many years because of their high speed and high sensitivity. A typical MSM photo-detector as shown in FIG. 1 comprises an absorbing layer of an appropriate thickness on a substrate with metal electrodes deposited on the absorbing layer. An exemplary MSM photo-detector for application in the 850 nm wave length range or lower comprises an absorbing layer of un-doped GaAs and a semi-insulating GaAs substrate. An example of MSM photo-detector is described in U.S. Pat. No. 5,461,246 which is incorporated herein by reference.